Update StringUtils.java

[inonwir]

Add md5FromFile to StringUtils for computing file checksums

• Adds a new method md5FromFile(String pathToResource) that computes the MD5 checksum of a file's contents.
• Complements the existing md5(String input) for strings.
• Inspired by a common StackOverflow pattern for file hashing.

[Jackckr]

来信已收到，谢谢~~